Had the same problem with 2.0 idci in traffic it was the egr sticking open,Just cleaned it out or you could fit a blank plate.
http://www.ebay.co.uk/itm/FORD-FIESTA-FUSION-FOCUS-FIESTA-B-C-MAX-EGR-VALVE-BLANKING-PLATE-1-5MM-STEEL-NZ-/172131321876?hash=item2813d38814:g:uXcAAOSwu1VW5AQx

The new software is 1412 or it was in march when i did mine.Had no problems with it.You need something like this http://www.ebay.co.uk/itm/111854666064?_trksid=p2060353.m1438.l2661&ssPageName=STRK%3AMEBIDX%3AIT to do the update. Sorry dont know how you will go on with a aftermarket stereo.

Check the current software by holding in Ok on the steering wheel as you turn the car on and yes the coolant display is in one of the menus on the old software (1218)the new software is 1412. Email alex ( murphy@yandex.ru ) for updates he has done a lot of work on the convers+ and has put the vids on youtube